1. Field of the Invention
This invention relates to the recovery of ethylene oxide from aqueous solutions containing the same and, more specifically, to an improved recovery system in which ethylene oxide is separated from an impure aqueous solution containing aldehydic impurities.
2. Description of the Prior Art
Ethylene oxide, a staple in commerce, is prepared by the industry in large quantities by oxidizing ethylene with air or elemental oxygen over a suitable catalyst, typically a silver-containing catalyst, at elevated temperature (100.degree. C. to 500.degree. C. is typical) and at superatmospheric pressure (2 to 25 atmospheres), e.g., by the process of U.S. Pat. No. 2,775,510.
The dilute ethylene oxide mixtures obtained from these reactions, which may be suitably conducted in fixed or fluid-bed reactors, are scrubbed with water to form an aqueous solution of ethylene oxide and to thereby separate the ethylene oxide from unreacted ethylene, oxygen and other gaseous components of the reaction mixture (e.g., carbon dioxide). The separated gaseous materials are generally recycled to the catalytic oxidation step. The aqueous ethylene oxide solution is withdrawn from the absorber and passed to a stripper, e.g., stripping column. In the stripper, generally steam is introduced, usually countercurrently to the ethylene oxide solution fed thereto, to remove ethylene oxide product as overhead. An aqueous stream containing small quantities of formaldehyde and ethylene oxide is withdrawn from the stripper as bottoms and is recirculated to the absorber for use in absorbing additional ethylene oxide.
The overhead product from the stripping column, containing CO.sub.2, ethylene oxide, gaseous inerts and water vapor, is cooled to partially condense the ethylene oxide and water contained therein, and the resulting mixture of vapor and liquid is passed to an ethylene oxide reabsorber, in which the uncondensed ethylene oxide vapor is reabsorbed in water. A predominance of the carbon dioxide and gaseous inerts which remain unabsorbed are readily separated as gaseous overhead stream from this reabsorption step. An aqueous solution is thus obtained which contains the reabsorbed ethylene oxide and aldehydic impurities, such as formaldehyde and acetaldehyde, as well as dissolved carbon dioxide and other gaseous impurities, and which must be further treated to provide the high purity ethylene oxide required by the industry. In the processes of U.S. Pat. Nos. 3,165,539, 3,174,262, and 3,964,980, this aqueous stream is passed to a "refining column" in which ethylene oxide is recovered as overhead and an aqueous bottoms is withdrawn for recycle to the reabsorber. In some processes (e.g., U.S. Pat. No. 3,904,656), the ethylene oxide overhead from the refining column is further purified in a second distillation column to remove any remaining carbon dioxide as overhead, and ethylene oxide bottoms are obtained which are passed to a third distillation column wherein purified ethylene oxide product is recovered as overhead.
While the above methods produce an ethylene oxide which is substantially free of water, carbon dioxide and dissolved inert gases, these methods have not economicaly dealt with the low concentrations of aldehydic impurities such as formaldehyde and acetaldehyde which are present in the ethylene oxide sought to be purified.
For example, while the prior art has typically removed formaldehyde as an overhead bleed in the purification step following reabsorption of the ethylene oxide from the stripping column, this has several disadvantages. If the formaldehyde concentration in the overhead bleed is high, a solid paraformaldehyde phase can form in the overhead system of the column which can result in blockage and erratic operation and can possibly require shutdown and cleanout. See, e.g., J. Frederic Walker, Formaldehyde, pgs. 140-163 (3d Ed. Reinhold Publishing Corp.). On the other hand, if the overhead bleed contains a low formaldehyde concentration, the relative amount of ethylene oxide therein is excessive and this results in yield loss of desired purified material.
While U.S. Pat. No. 3,418,338 offers a process which provides more efficient removal of formaldehyde, the attendant disadvantage of the presence of acetaldehyde is not completely solved by this process, and separation of these aldehydic impurities to provide a more pure ethylene oxide stream without the use of condensation and further distillation required by this process would be advantageous from a cost standpoint.